Repeated attempts to upgrade to the new MobileMe Calendar received the same error as Richard_B.:
"The calendar
https://cal.me.com/principals/users/[my ID]/ was not found on the server. Make sure the URL is correct."
Software is up-to-date on 27" Intel iMac (10.6.5) and all iOS devices (4x). Deleted my MobileMe calendar account from appropriate iCal prefs pane and now no longer receive above error. Calendar changes to iCal, MobileMe Calendar, and iOS device calendars still push to populate other calendar instances.
Still, I desire the new MobileMe calendar primarily for its compatibility with BusyMac's BusyToDo mobile app, which allows to-do list viewing and editing on iOS devices. Working with a very simple calendaring system "On My Mac": distinct "Home" and "Work" calendars, as well as a third one. "Subscribed" to a Birthday calendar, and that's it. No calendar categories involved.
MobileMe syncing (iDisk and iCal particularly) has been very spotty over many years. BusyCal or some other alternative for calendaring beckons, as does DropBox. Always resisted third-party solutions for syncing, but feel myself caving.